Process for preparing absorbent resin

Abstract: A process for preparing an absorbent resin includes the steps of: preparing a first crosslinking agent (A) including at least two polymerizable unsaturated bonds; preparing a second crosslinking agent (B) including at least two polymerizable unsaturated bonds and hydroxyl groups; preparing a crosslinking agent including (A) and (B) with a ratio of (A) to (B) ranging from 95/5 to 50/50; and polymerizing a hydrophilic unsaturated monomer having at least one carboxyl group using the crosslinking agent. The crosslinking agent includes a crosslinking agent which includes a main component at least 90 percent by weight, the main component being an ester having at least two polymerizable unsaturated bonds, wherein the ester is composed of a polyhydroxy alcohol (C) having not more than six carbon atoms and at least three hydroxy groups, and an unsaturated carboxylic acid (D); and a molecule weight ratio of the main component to a standard compound (E) is in the range of from 0.7/1 to less than 1.3/1, (E) being an ester composed of (C) that composes the main component, and (D) that composes the main component, in which all the hydroxy groups in (C) are ester-linked with (D).

Description: FIELD OF THE INVENTION The present invention relates to a process for preparing an absorbent resin, more specifically relates to a process for preparing an absorbent resin which exhibits excellent properties including its gel stability, diffusivity of aqueous liquid and permeability to aqueous liquid, suitable for use in sanitary goods. BACKGROUND OF THE INVENTION Recently, an absorbent resin is widely used in sanitary goods such as paper diaper, sanitary napkin, etc., for the purpose of absorbing liquid. Examples of the conventionally known absorbent resins includes: a partially neutralized crosslinked polymer of polyacrylic acid (Japanese Unexamined Patent Publication No. 84304/1980 (Tokukaisho 55-84304), Japanese Unexamined Patent Publication No. 108407/1980 (Tokukaisho 55-108407) and Japanese Unexamined Patent Publication No. 133413/1980 (Tokukaisho 55-133413)); a hydrolyzed polymer of starch-acrylonitrile (Japanese Unexamined Patent Publication No. 43995/1971 (Tokukaisho 46-43995)); a neutralized graft polymer of starch-acrylic acid (Japanese Unexamined Patent Publication No. 125468/1976 (Tokukaisho 51-125468)); a saponified copolymer of vinyl acetate-acrylic ester (Japanese Unexamined Patent Publication No. 14689/1977 (Tokukaisho 52-14689)); a hydrolyzed copolymer of acrylonitrile or of acrylamide (Japanese Unexamined Patent Publication No. 15959/1978 (Tokukaisho 53-15959)); a crosslinked polymer of cationic monomer (Japanese Unexamined Patent Publication No. 154709/1983 (Tokukaisho 53-154709) and Japanese Unexamined Patent Publication No. 154710/1983 (Tokukaisho 58-154710)). Among the above-listed crosslinked polymers, a crosslinked polymer obtained by polymerizing a hydrophilic unsaturated monomer having at least one carboxyl group such as acrylic acid in the presence of a crosslinking agent is the most common material for the absorbent resin, because the raw material for the above polymer is available at low price, while exhibiting excellent absorbent properties without the problem of putrefaction. The absorbent resin desirably exhibits the following properties when it is in contact with aqueous liquid: high magnifier and rate of absorbency; an excellent stability of its swollen gel; high diffusivity of and permeability to aqueous liquid, a strong suction power for sucking aqueous liquid from the material containing aqueous liquid, etc. Recently, still thinner sanitary goods of higher quality have been demanded, and an amount of use of the absorbent resin has been increasing. Thus, an absorbent resin which exhibits improved gel stability, diffusivity of and permeability to aqueous liquid is strongly demanded. As well known, these properties of the absorbent resin are greatly affected by a crosslinking agent used in the preparation process of the absorbent resin. The crosslinking agent used in the preparation of the absorbent resin must be commercially available and safe, and it is also required to exhibit excellent affinity and polymerizability with a hydrophilic unsaturated monomer. For the above mentioned absorbent resin, unsaturated carboxylates of polyhydroxy alcohol, which have a plurality of polymerizable unsaturated groups and are polymerizable with a hydrophilic unsaturated monomer having at least one carboxyl group may be used. Examples of the above unsaturated carboxylates include: di(meth)acrylate of polyoxyalkylene glycol, (poly)propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexamethacrylate, and the like. In order to prepare the absorbent resin which exhibits an excellent gel stability and diffusivity of aqueous liquid using one of the above-listed unsaturated carboxylates as a crosslinking agent, the crosslinking agent must be used in a large amount. Here, the amount of use of the crosslinking agent and the gel stability have a virtually positive relationship. However, because of other factors, such as copolymerizability, affinity and solubility of the crosslinking agent and the hydrophilic unsaturated monomer having at least one carboxyl group, etc., the reactivity of the crosslinking agent is lowered when it is used in an excess amount of a predetermined range. Namely, the excessive amount of the crosslinking agent would not give an improvement in the above properties of the absorbent resin as much as expected for the amount of use of the crosslinking agent. Moreover, the effect of reducing the water soluble polymer content, which affects the diffusivity of and permeability to aqueous liquid of the absorbent resin, is reduced. Thus, in order to obtain absorbent resin which exhibits an excellent gel stability, a large amount of the crosslinking agent is required. In order to improve the copolymerizability, affinity and solubility of the crosslinking agent and the hydrophilic unsaturated monomer, for example, a method for polymerizing in the presence of a surface active agent (Japanese Examined Patent Publication No. 36763/1986 (Tokukosho 61-36763)) and a method for polymerizing in the presence of a dispersant (Japanese Unexamined Patent Publication No. 146902/1989 (Tokukohei 1-146902) are disclosed. However, since all of the above methods present the problem that the surface active agent or the dispersant remains in the resulting absorbent resin, which lowers the absorbent properties of the resulting absorbent resin, they are not desirable. SUMMARY OF THE INVENTION An object of the present invention is to provide a process for preparing an absorbent resin which exhibits excellent properties including its gel stability, diffusivity of aqueous liquid and permeability to aqueous liquid with low water soluble polymer content, which are suitable for use in sanitary goods, without having therein by-products such as a surface active agent or dispersant remaining as residues nor requiring a large amount of crosslinking agents. In order to achieve the above object, inventors of the present invention examined a process for preparing the absorbent resin with respect to polymerizing a hydrophilic unsaturated monomer having at least one carboxyl group in the presence of a crosslinking agent having at least two polymerizable unsaturated bonds, and discovered that by mixing or combining the crosslinking agent having at least two polymerizable unsaturated bonds at a predetermined ratio with another crosslinking agent having at least two polymerizable unsaturated bonds and hydroxy groups, the above-mentioned problems would be solved. The inventors of the present invention also discovered that in the case of using an ester of a polyhydroxy alcohol having at least three hydroxy groups and an unsaturated carboxyl acid as the crosslinking agent, if more than a predetermined amount of a high boiling-point compound having at least two polyhydroxy alcohol structures in a molecule is included, it would be difficult to achieve the improved absorbent properties of the absorbent resin. This means that the above problem can be prevented also by adjusting the content of the high boiling-point compound present in the crosslinking agent. In order to achieve the above object, the process for preparing the absorbent resin in accordance with the present invention is characterized by comprising the steps of: preparing the first crosslinking agent including at least two polymerizable unsaturated bonds; preparing the second crosslinking agent including at least two polymerizable unsaturated bonds and hydroxy groups; preparing a crosslinking agent including the first crosslinking agent and the second crosslinking agent with a ratio of the first crosslinking agent to the second crosslinking agent ranging from 95/5 to 50/50; and polymerizing a hydrophilic unsaturated monomer having at least one carboxyl group using the crosslinking agent. The above process gives improved copolymerizability, affinity and solubility, between the crosslinking agent and the hydrophilic unsaturated monomer and gives a simple process for preparing the absorbent resin which exhibits improved properties including stability and strength of the gel, and the diffusivity of and permeability to aqueous liquid with low water soluble polymer content, suitable for use in sanitary goods without having therein by-products such as a surface active agent or dispersant therein as residues nor requiring a large amount of the crosslinking agent. The following descriptions will describe the present invention in details. Although the hydrophilic unsaturated monomer having at least one carboxyl group used as a raw material for the absorbent resin of the present invention is not specified, examples of the hydrophilic unsaturated monomer include: acrylic acid, methacrylic acid, maleic acid, maleic anhydride, β-acryloxypropionic acid, fumaric acid, crotonic acid, itaconic acid, sorbic acid, cinnamic acid or the above acids in a neutralized form. The above listed hydrophilic unsaturated monomer may be one of the above listed acids or a combination or mixture thereof. Among the above-listed acids, the acrylic acid, methacrylic acid and these acids in a neutralized form are particularly preferable. Generally, the affinity and solubility between the hydrophilic unsaturated monomer and the crosslinking agent is lowered when the carboxyl groups in the monomer are neutralized. On the contrary, the advantages of the process according to the present invention become particularly clear when the carboxyl groups in the monomer are neutralized. Therefore, it is particularly preferable that the acrylic acid and the methacrylic acid be present in the partially neutralized form in the range from 30 mole percent to 90 mole percent of all carboxyl groups, with a more preferred range being from 50 mole percent to 80 mole percent and the most preferred range being from 60 mole percent to 75 mole percent. The above hydrophilic unsaturated monomer used as a raw material for the absorbent resin may be combined or mixed with another unsaturated monomer if necessary. Although the material for this additionally used monomer is not specified in the present invention, for example, the following materials may be used: an anionic unsaturated monomer such as vinylsulfonic acid, styrenesulfonic acid, 2-(meth)acrylamide-2-metylpropanesulfonic acid, 2-(meth)acryloylethanesulfonic acid, 2-(meth)acryloylpropanesulfonic acid, and salts thereof; a nonionic unsaturated monomer including hydrophilic groups such as acrylamide, methacrylamide, N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide, 2-hidroxylethyl(meth)acrylate, 2-hydroxylpropyl(meth)acrylate, methoxypolyethylene glycol (meth)acrylate, polyethylene glycol mono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, and N-acryloylpyrrolidine; a cationic unsaturated monomer such as N,N-dimethylaminoethyl(meth)acrylate, N,N-diethylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl(meth)acrylate, N,N-diethylaminopropyl(meth)acrylate and quaternary salts thereof. The above additional hydrophilic unsaturated monomer is preferably is used in an amount not more than 50 percent by weight to the total weight of the hydrophilic unsaturated monomers including the original hydrophilic unsaturated monomer having at least one carboxyl group. As described, the crosslinking agent used in the polymerization in the process for preparing the absorbent resin of the present invention includes the first crosslinking agent having at least two polymerizable unsaturated bonds and the second crosslinking agent having at least two polymerizable unsaturated bonds and at least one hydroxy group at a ratio of the first crosslinking agent to the second crosslinking agent ranging from 95/5 to 50/50. The first crosslinking agent may be at least one material selected from the group consisting of N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,5-pentanediol (meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol diacrylate, trimethylolpropane tri(meth)acrylate, ethylene oxide denatured trimethylolpropane tri(meth)acrylate, glycerin tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, ditrimethylolpropane tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, tetramethylolmethane tetraacrylate, triallyl cyanurate, triallyl isocyanurate, triallylamine and tetraaryloxyethane. For the first crosslinking agent, those exhibiting low affinity and solubility with the hydrophilic unsaturated monomer having at least one carboxyl group (especially in a neutralized form) is preferable because the advantages of the process according to the present invention become clear. The first crosslinking agent preferably includes at least three polymerizable unsaturated bonds in consideration of the gel strength and the water soluble polymer content of the resulting absorbent resin. The first crosslinking agent may be at least one material selected from the group consisting of trimethylolpropane tri(meth)acrylate, ethylene oxide denatured trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, triarylamine and tetraaryloxy ethane. The second crosslinking agent may be at least one material selected from the group consisting of trimethylolpropane di(meth)acrylate, ethylene oxide denatured trimethylolpropane di(meth)acrylate, glycerin acrylate methacrylate, glycerin di(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol penta(meth)acrylate, tetramethylolmethane triacrylate, reaction product of polyhydroxy epoxy compound and (meth)acrylic acid. The second crosslinking agent preferably has the similar structure to the first crosslinking agent and exhibits an excellent affinity with the first crosslinking agent because improved affinity and the solubility of the first crosslinking agent with hydrophilic unsaturated monomer having at least one carboxyl group (especially in a neutralized form) can be achieved. The second crosslinking agent in accordance with the present invention is preferably at least one material selected from the group consisting of trimethylolpropane di(meth)acrylate, ethylene oxide denatured trimethylolpropane di(meth)acrylate and pentaerythritol tri(meth)acrylate. As a typical example of the first crosslinking agent and the second crosslinking agent, an ester of polyhydroxy alcohol having not more than six carbon atoms and at least three hydroxy groups and an unsaturated carboxylic acid may be mentioned. Here, if the first crosslinking agent and the second crosslinking agent have the same polyhydroxy alcohol structure, it would not be necessary to prepare the two crosslinking agents separately. As long as the ratio of the compound of the first crosslinking agent to the compound of the second crosslinking agent falls within a predetermined range of the present invention by adjusting the conditions for the esterification process, a compound resulting from a sole esterification may be used directly as the crosslinking agent in the process for preparing the absorbent resin of the present invention. In the esterification process, a high boiling-point compound having at least two polyhydroxy alcohol structures in a molecule having at least three hydroxy groups in the molecule may be generated as a by-product. However, as long as the ratio of the first crosslinking agent to the second crosslinking agent in the total crosslinking agent falls within the predetermined range, the high boiling-point compound may be included in a range of from above 0 percent by weight to less than 30 percent by weight with a more preferred range being from 10 percent by weight to 20 percent by weight, most preferably less than 10 percent by weight. Further preferred embodiments are described in the dependent claims. In the process for preparing an absorbent resin of the present invention, the ratio of the first crosslinking agent to the second crosslinking agent is set in a range of from 95/5 to 50/50. When the second crosslinking agent is used in an amount lower than the above mentioned range, the desirable affinity and solubility between the first crosslinking agent in the relatively large amount and the monomer are difficult to be achieved. Moreover, improvements in the other properties of the absorbent resin such as the gel strength and the water soluble polymer content are also difficult to be achieved. On the other hand, when the second crosslinking agent is used in an excess amount of the above mentioned range, the properties of the first crosslinking agent used in a relatively small amount are hardly shown, and the gel strength may be lowered or the physical properties of the absorbent resin such as the magnifier of the absorbency may not be controlled. Therefore, the ratio of the first crosslinking agent to the second crosslinking agent is preferably set in a range of from 90/10 to 60/40. Additionally, it is preferable that the total amount of the crosslinking agent is set in a range of from about 0.01 to 5 mole percent, preferably 0.05 mole percent to 1 mole percent with respect to the hydrophilic unsaturated monomer having at least one carboxyl group. In the described process for preparing an absorbent resin of the present invention, the crosslinking agent used in the polymerization process may be combined with a small amount of another crosslinking agent (hereinafter referred to as first auxiliary crosslinking agent) as long as the advantages of the process according to the present invention can be achieved. Examples of the first auxiliary crosslinking agent include: (poly)ethylene glycol diglycidyl ether, glycerol diglycidyl ether, ethylene glycol, polyethylene glycol, propylene glycol, glycerin, pentaerythritol, ethylene carbonate, propylenecarbonate, ethylenediamine, polyethyleneimine and glycidyl (meth)acrylate. Additionally, a water soluble polymer such as starch, cellulose, derivatives thereof, polyacrylic acid and polyvinyl alcohol, may be present in the aqueous solution of the hydrophilic unsaturated monomer in order to obtain a graft copolymer. In polymerizing the hydrophilic unsaturated monomer having at least one carboxyl group in the presence of the crosslinking agent in the present invention, a bulk polymerization or precipitation polymerization may be employed. However, in consideration of the performance, the easiness of the control of polymerization, an aqueous polymerization for polymerizing the hydrophilic unsaturated monomer in an aqueous solution or a reverse-phase suspension polymerization are especially preferable, and the aqueous polymerization for polymerization in the aqueous solution without using an organic solvent is especially preferable. In this polymerization process, the hydrophilic unsaturated monomer is present in the aqueous solution in an amount ranging from 20 percent by weight to a saturation concentration, with a more preferred range being from 25 percent by weight to 40 percent by weight in order to make the advantages of the process according to the present invention greater. In the polymerization process of the present invention, the following polymerization initiator may be used: a radical polymerization initiator such as potassium persulfate, ammonium persulfate, sodium persulfate, t-butyl hydroperoxide, hydrogen peroxide, 2,2'-azobis(2-aminodipropane) dihydrochloride, an active energy rays such as ultraviolet rays, electron rays, and the like. In the case of employing an oxydative radical polymerization initiator, a redox polymerization may be carried out by simultaneously using a reducing agent such as sodium sulfite, sodium bisulfite, ferrous sulfate, L-ascorbic acid. In order to enable the resulting absorbent resin to be treated as particles, water content of the absorbent resin is set in a range of from 0.1 percent to 50 percent with a more preferred range being from 1 percent to 20 percent and the most preferred range being from 1 percent to 10 percent. Normally, the polymerization process is followed by a drying process. In the process for preparing an absorbent resin of the present invention, the polymer resulting from the polymerization process is preferably heated at a temperature in the range of from 80 °C to 250 °C with a more preferred range being from 150 °C to 220 °C. A heating temperature is set in the above range because if the polymer is heated at temperature above 250 °C, the properties of the absorbent resin may be lowered and if the polymer is heated at a temperature below 80 °C, a sufficient gel strength may not be achieved. For the heating process, a generally used dryer or heater may be used such as a hot-air dryer, thin-type mix dryer, rotary dryer, disk dryer, fluidized-bed dryer, flash dryer and infra-red dryer. The resulting absorbent resin may have, for example, undefined, circular, fibrous, stick-like, or virtually circular shape. However, in consideration of the applications thereof to the sanitary goods with respect to the diffusivity of aqueous liquid and the resistance to moving out of the pulp, it is preferable that the absorbent resin has an undefined shape with a preferred range of an average particle diameter being from 10 µm to 1000 µm and a more preferred range of the average particle diameter being from 300 µm to 600 µm. In the present invention, in order to obtain a still improved absorbent properties of the resulting absorbent resin under pressure, such as a magnifier of absorbency, absorbing power, permeability to aqueous liquid, etc., the absorbent resin resulting from the process of the present invention may be mixed with a crosslinking agent which is reactive with a carboxyl group (hereinafter referred to as second auxiliary crosslinking agent) and heated as the vicinity of the surface of the absorbent resin is more strongly crosslinked. Examples of the second auxiliary crosslinking agent include the following known crosslinking agents: polyhydroxy alcohol compounds such as ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, propylene glycol, 1,3-propanediol, dipropylene glycol, 2,2,4-trimethyl-1,3-pentadiol, polypropylene glycol, glycerin, polyglycerin, 2-butane-1,4-diol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexanedimethanol, 1,2-cyclohexanol, trimethylolpropane, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene oxopropylene block copolymer, pentaerythritol and sorbitol; polyhydroxy epoxy compounds such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol polyglycidyl ether, diglycerol polyglycidyl ether, polyglycerol polyglycidyl ether, propylene glycol diglycidyl ether and polypropylene glycol diglycidyl ether; polyamine compounds such as ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, pentaethylenehexamine and polyethyleneimine; polyhydroxy isocyanate compounds such as 2,4-tolylene diisocyanate and hexamethylene diisocyanate; polyhydroxy oxazoline compounds such as 1,2-ethylenebisoxazoline; alkylene carbonate compounds such as 1,3-dioxolane-2-one, 4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one, 4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one, 4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, 1,3-dioxopane-2-one, etc.; haloepoxy compounds such as epichlorohydrin, epibromehydrine and α-methylepichlorohydrin; metallic compounds such as hydroxides and chlorides of metals: zinc, calcium, magnesium, aluminum, iron and zirconium. In blending the second auxiliary crosslinking agent for crosslinking the vicinity of the surface, it is preferable that water is added either simultaneously or separately, and an insoluble particle powders, surface active agent and hydrophilic organic solvent, may be used as well. The first and second processes of the present invention for preparing the absorbent resin give improved properties of the absorbent resin: its gel stability and water soluble component content. Although the factors which give the improvement of the above properties are not sure, the following is presumed to be the reason why the improved properties of the absorbent resin can be achieved: the affinity and solubility of the crosslinking agent to the hydrophilic unsaturated monomer having at least one carboxyl group can be improved significantly by the combined use of the second crosslinking agent and by adjusting the content of a high boiling-point compound, and thus a uniform crosslinking structure can be achieved. Additionally, a deodorant, perfume, inorganic powder, foaming agent, pigment, dye, hydrophilic short fiber, synthetic fiber, fertilizer, oxidizing agent, reducing agent, water and salts, may be added to the resulting absorbent resin prepared by the process of the present invention, which enables the absorbent resin to have various functions and thus applicable to other fields than the sanitary goods such as paper diaper and sanitary napkin. As described, the process in accordance with the present invention enables the absorbent resin to be prepared without having therein by-products such as the surface active agent and diffusants, nor using a large amount of crosslinking agent. Moreover, the resulting absorbent resin prepared by the described process according to the present invention exhibits excellent properties including its gel stability, water soluble component content, diffusivity of and permeability to aqueous liquid, suitable for use in sanitary goods. The absorbent resin prepared by the process of the present invention is suitable for use in sanitary goods such as paper diaper, sanitary napkin even when it is used in a larger amount. EXAMPLES The following examples and comparative examples are presented for the purpose of illustration only, and are not to be construed as limiting the scope of the invention in any way. The properties of the resulting absorbent resins were measured in manners presented below. Example 1 (a) absorbency In a tea bag (40 mm × 150 mm) made of a nonwoven material, 0.2 g of absorbent resin was uniformly fed, and the tea bag was put in 0.9 percent by weight of sodium chloride (physiologic saline) solution. After leaving the tea bag in the solution for 30 minutes, it was taken out and dewatered for a predetermined time. Then, the tea bag was weighted W 1 . Another tea bag was prepared in the same manner without using the absorbent resin, and the above process was followed. Then, the tea bag was weighted W 0 . Using the obtained two weights W 0 and W 1 , the absorbency (g/g) of the absorbent resin was calculated through the following equation: absorbency (g/g) = weight W 1 (g) - weight W 0 (g) absorbent resin weight (g) (b) gel strength To 100 ml of synthetic urine, 1.0 g of absorbent resin was added, and after leaving it for an hour, the mixture was filtered off through a wire netting so as to obtain a swollen gel. Then, after extracting an excessive liquid from the swollen gel using paper, the swollen gel was set in a viscosity measuring unit (FLUID SPECTROMETER MODEL 8400 available from the Rheometric Co.) so as to measure a shearing modulus (kdyn/cm 2 ), which represented the gel strength. (c) gel stability In a 100 ml propylene container (diameter: 50mm) with a cover, 2.0 g of absorbent resin was fed and urine collected from five adult persons was added so as to swell the absorbent resin. The container was closed with the cover and left for six hours at 40 °C. Then, the container was inclined to a horizontal position and the distance (mm) the gel moved in one minute was measured, which represented the gel stability. Here, the shorter the distance the gel moves, the more stable the swollen gel is in respect to the urine. (d) water soluble component content In 1000 ml of deionized water, 0.500 g of absorbent resin was diffused, and after agitating it for 16 hours, the mixture was filtered off through a filter paper, and the water soluble component content (percent) was calculated by a colloidal titration. The crosslinking agent for the process of the present invention was prepared in the following manner. Preparation 1 In an one-liter flask with four openings provided with an agitator, water-separator with a cooling tube, thermometer and air-introducing tube, 134 g of trimethylolpropane, 238 g of acrylic acid, 170 g of toluene, 24 g of p-toluenesulfonic acid and 0.6 g of hydroquinone were fed, and the flask was heated to 130 °C while introducing air thereinto. Produced water was distilled off from a reaction system by an azeotropic dehydration with toluene, while observing the reaction by a gas chromatography (GC), and when the ratio by weight of the trimethylolpropane triacrylate to trimethylolpropane diacrylate reached 85/15, the reaction was stopped. Then, the reaction solution was transferred to a separating funnel, and after neutralizing the unreacted acrylic acid with 500 g of 10 percent NaOH aqueous solution and 300 g of 5 percent NaOH aqueous solution, the reaction solution was layered and washed with water (500 g each) several times until the washed liquid was neutralized. Then, to the organic layer, 0.06 g of hydroquinone monomethyl ether was added, and the toluene was distilled under reduced pressure, thereby obtaining a reaction product. The reaction product was analyzed using the GC, and the result of the calculation based on their peak area ratio gave the ratio of the trimethylolpropane triacrylate and trimethylolpropane diacrylate of 88/12 (by weight). The reaction product was also analyzed using a gel permeation chromatography (GPC). As a result a high boiling-point compound including at least two trimethylolpropane structures was contained 14 percent by weight. Using the resulting crosslinking agent from Preparation 1, the hydrophilic unsaturated monomer having at least one carboxyl group was polymerized in the following manner: In 414 g of acrylic acid (hydrophilic unsaturated monomer), 4. 77 g of a crosslinking agent resulting from Preparation 1 was dissolved. To the mixture, 4382 g of 37 percent by weight of sodium acrylate and 699 g of ion exchange solution were added to give a reaction solution. In a stainless steel reactor of two-arms type kneader with a cover equipped with a jacket (volume content: 10 L) with two sigma blades, the reaction solution was poured, and the reaction system was displaced by introducing nitrogen gas while maintaining the reaction solution at 30 °C. Then, while agitating the reaction solution maintained at 30 °C, 2.40 g of sodium persulfate and 0.12 g of L-ascorbic acid were added to the reaction solution so as to start the polymerization. After leaving it for 60 minutes, a resulting finely divided water-containing gel-like polymer was taken out. The resulting finely divided water-containing gel-like polymer was placed on a wire netting of 50 mesh and dried under hot air at 150 °C for 90 minutes. Then, the resulting dried polymer was pulverized by a vibrating mill and further classified by a wire netting of 20 mesh. As a result, an absorbent resin of undefined shape with a water content of 8 percent and an average particle diameter of 370 µm was obtained. The absorbency, gel strength, gel stability, water soluble component content (hereinafter simply referred to as results) of the resulting absorbent resin are presented in Table 1. As is clear from the results, the absorbent resin resulting from Example 1 exhibits excellent properties: gel strength, gel stability, diffusivity of and permeability to aqueous liquid, and low water soluble component content. Example 2 The same preparation procedure and analysis as in Example 1 were conducted except that the amount of use of the crosslinking agent in Example 1 was altered to 3.41 g, and an absorbent resin was obtained. The results of Example 2 are presented in Table 1. Example 3 The same preparation procedure and analysis as in Example 1 were conducted except that the amount of use of the crosslinking agent in Example 1 was altered to 2.04 g, and an absorbent resin was obtained. The results of Example 3 are presented in Table 1. Example 4 Preparation 2 The same preparation procedure as in Preparation 1 was conducted except that the reaction was stopped when the ratio of trimethylolpropane triacrylate to trimethylolpropane diacrylate reached 58/42 (by weight). An obtained reaction product was analyzed using the GC, and the ratio of the methylolpropane triacrylate to trimethylolpropane diacrylate was calculated based on their peak area ratio, and the ratio of 62/38 (by weight) was obtained. The reaction product was also analyzed using a gel permeation chromatography (GPC). As a result, the reaction product includes 11 percent by weight of a high boiling-point compound including at least two trimethylolpropane structures. The same preparation procedure and analysis as in Example 1 were conducted except that the crosslinking agent resulting from Preparation 2 was used in an amount of 3.41 g, and an absorbent resin was obtained. The results of Example 4 are presented in Table 1. As is clear from the results of Examples 2 to 4, the absorbent resins resulting from Examples 2 to 4 exhibit excellent gel stability, low water soluble component content, and excellent diffusivity of and permeability to aqueous liquid as obtained from Example 1. Comparative Example 1 Preparation 3 The same preparation procedure as in Preparation 1 was conducted except that the reaction was stopped when the ratio of trimethylolpropane triacrylate to trimethylolpropane diacrylate reached 97/3 (by weight). The reaction product was analyzed using the GC, and the ratio of the methylolpropane triacrylate to trimethylolpropane diacrylate was calculated based on their peak area ratio, and the ratio of 98/2 (by weight) was obtained. The reaction product was also analyzed using a gel permeation chromatography (GPC). As a result a high boiling-point compound including at least two trimethylolpropane structures was contained 21 percent by weight. Using 4.77g of the crosslinking agent resulting from Preparation 3, the hydrophilic unsaturated monomer having at least one carboxyl group was polymerized as in the same manner as Example 1, and an absorbent resin was obtained. The results of Comparative Example 1 are presented in Table 1. Comparative Example 2 The same preparation procedure and analysis as in Example 1 were conducted except that the crosslinking agent resulting from Preparation 3 was used in an amount of 3.41 g, and an absorbent resin was obtained. The results of Comparative Example 2 are presented in Table 1. Comparative Example 3 The same preparation procedure and analysis as in Example 1 were conducted except that the crosslinking agent resulting from Preparation 3 was used in an amount of 2.04 g, and an absorbent resin was obtained. The results of Comparative Example 3 are presented in Table 1. The results of Examples 1 to 4 and Comparative Examples 1 to 3 show that when the crosslinking agent is used outside the range of the ratio of the first crosslinking agent to the second crosslinking agent defined in the present invention as in the case of Comparative Examples 1 to 3, the resulting absorbent resins exhibit inferior properties including the gel stability, water soluble component content, and the diffusivity of and permeability to aqueous liquid compared with the absorbent resins resulting from Examples 1 to 4. Example No. Weight Ratio of First to Second Crosslinking Agents Absorbency (g/g) Gel Strength (kdyn/cm 2 ) Gel Stability (mm) Water Soluble Component Content (%) 1 88/12 43 26 0 5 2 88/12 47 19 2 7 3 88112 51 15 10 10 4 62/38 46 19 2 5 Comparative Example No. 1 98/2 43 23 2 7 2 98/2 47 17 10 10 3 98/2 51 14 18 11

Claims: A process for preparing an absorbent resin comprising the steps of: preparing a first crosslinking agent including at least two polymerizable unsaturated bonds; preparing a second crosslinking agent including at least two polymerizable unsaturated bonds and hydroxyl groups; preparing a crosslinking agent including the first crosslinking agent and the second crosslinking agent with a ratio of the first crosslinking agent to the second crosslinking agent ranging from 95/5 to 50/50;and polymerizing a hydrophilic unsaturated monomer having at least one carboxyl group using the crosslinking agent. The process according to claim 1, wherein both the first crosslinking agent and the second crosslinking agent are esters of polyhydroxy alcohols having not more than six carbon atoms and at least three hydroxy groups, and unsaturated carboxylic acid. The process according to claim 1, wherein the first crosslinking agent includes at least one material selected from the group consisting of trimethylolpropane triacrylate, ethylene oxide denatured trimethylolpropane triacrylate, pentaerythritol tetraacrylate, triarylamine and tetraaryloxyethane. The process according to claim 1, wherein the second crosslinking agent includes at least one material selected from the group consisting of trimethylolpropane diacrylate, ethylene oxide denatured trimethylolpropane diacrylate and pentaerythritol triacrylate. The process according to claim 2, wherein the crosslinking agent includes a high boiling-point compound at a proportion of not more than 30 percent by weight, the high boiling-point compound including at least two polyhydroxy alcohol structures having at least three hydroxy groups in a molecule, or wherein the crosslinking agent includes a high boiling-point compound at a proportion range of from 10 percent by weight to 20 percent by weight, the high boiling-point compound including at least two polyhydroxy alcohol structures having at least three hydroxy groups in a molecule, or wherein the crosslinking agent consists of only a compound including one polyhydroxy alcohol structure having at least three hydroxy groups in a molecule, or wherein the crosslinking agent includes a high boiling-point compound at a proportion in the range of less than 10 percent by weight, the high boiling-point compound including at least two polyhydroxy alcohol structures in a molecule, or wherein the crosslinking agent includes its main component at least at 95 percent by weight, or wherein the crosslinking agent consists of only a compound including one polyhydroxy alcohol structure in a molecule, or wherein the crosslinking agent has a viscosity in the range of from 10 mPas (cps) to 60 mPas (cps) at 25 °C. The process according to claim 1, wherein the hydrophilic unsaturated monomer is a material selected from the group consisting of acrylic acid, methacrylic acid, acrylic acid in a neutralized form and methacrylic acid in a neutralized form. The process according to claim 1, wherein 30 mole percent to 90 mole percent of the carboxyl groups in the hydrophilic unsaturated monomer are neutralized, or wherein 50 mole percent to 80 mole percent of the carboxyl groups in the hydrophilic unsaturated monomer are neutralized, or wherein 60 mole percent to 75 mole percent of the carboxyl groups in the hydrophilic unsaturated monomer are neutralized. The process according to claim 1, wherein the process for polymerizing the hydrophilic unsaturated monomer is carried out in an aqueous solution for the hydrophilic unsaturated monomer having a concentration ranging from 20 percent by weight to saturation, or wherein the process for polymerizing the hydrophilic unsaturated monomer is carried out in an aqueous solution for the hydrophilic unsaturated monomer having a concentration ranging from 25 percent by weight to 40 percent by weight. The process according to claim 1, further comprising the step of heating a polymer of the hydrophilic unsaturated monomer having at least one carboxyl group at a temperature in the range of from 80 °C to 250 °C, or further comprising the step of heating a polymer of the hydrophilic unsaturated monomer having at least one carboxyl group at a temperature in the range of from 150 °C to 220 °C.

Inventors: HARADA NOBUYUKI, HIROTA KOHICHI, IRIE YOSHIO, KUBO HIDEYUKI, MASUDA YOSHIHIKO, NAGANO HIDEAKI, NAGASUNA KINYA, HARADA, NOBUYUKI, HIROTA, KOHICHI, IRIE, YOSHIO, KUBO, HIDEYUKI, MASUDA, YOSHIHIKO, NAGANO, HIDEAKI, NAGASUNA, KINYA
Applicants: NIPPON CATALYTIC CHEM IND, NIPPON SHOKUBAI CO., LTD.
Application year: 1994
Main classification: C08F220
IPC codes: C08F220, A61L15